To allow the reservoir containing either the fruit syrup, the concentrate or the like to be arranged at a height independent of the height of the mixing chamber and to ensure reliable function and adjusting the conduit supplying the mixing chamber is usually associated with a non-return valve and/or a pressure reducing mechanism. Either a spring-loaded non-return valve, a ball valve or a reducing valve may be employed in this case. This arrangement is relatively time-consuming to fabricate and expensive and difficulties arise from the danger of clogging and with regard to the cleaning which cannot be dispensed with in systems of such type. Despite the comparatively high manufacturing cost a conventional non-return valve can fulfill only one single function, namely can either close or open the supply conduit for syrup or concentrate in dependance upon the pressure conditions which occur.